1. Field of the Invention
The present invention relates to a device and a method for measuring image density, and more particularly, to a device and a method for measuring image density of liquid toner. The present application is based on Korean Patent Application No. 2001-61324, filed on Oct. 5, 2001, which is incorporated herein by reference.
2. Description of the Prior Art
Generally, a photosensitive medium such as a photosensitive belt or a photosensitive drum is used in an image forming apparatus. The surface of the photosensitive medium can be charged. Moreover, an electrostatic latent image can be formed on the surface of the photosensitive medium as an electric potential level is selectively converted by the projection of a beam. The image forming apparatus is divided into a dry-type and a wet-type according to the type of toner applied to the electrostatic latent image. The wet-type image forming apparatus uses a developing solution in which a liquid toner and a carrier are mixed.
FIG. 1 is a view schematically showing a conventional image forming apparatus.
Referring to FIG. 1, the image forming apparatus has: a photosensitive belt 14 supported by a driving roller 11, a steering roller 12, a counter roller 13 and rotated on a caterpillar; a plurality of developers 17a, 17b, 17c and 17d; and a transferring roller 25.
The surface of the photosensitive belt 14 can be maintained in a charged state by a corona device 16. The charged state of the surface of the photosensitive belt 14 can be changed by a laser beam projected from a plurality of laser projection units 18a, 18b, 18c and 18d disposed close to the developers 17a, 17b, 17c and 17d. The beam projection of the laser projection units 18a, 18b, 18c and 18d forms the electrostatic latent image on the photosensitive belt 14.
The developers 17a, 17b, 17c and 17d can develop colors such as black, yellow, cyan and magenta according to the pigment color. Moreover, each of the laser projection units 18a, 18b, 18c and 18d forms a part of the electrostatic latent image in regard to a corresponding color. The developers 17a, 17b, 17c and 17d, are filled with developing solution, which can be applied to the electrostatic latent image formed on the surface of the photosensitive belt 14 by a plurality of development rollers 22a, 22b, 22c and 22d respectively disposed at the developers 17a, 17b, 17c and 17d. The developing solution attached to the electrostatic latent image forms test patches on a printing paper 27. The test patches constitute a visible image and the test patches have a tone area coverage different from each other and different from a half tone.
A cleaning roller 23 has a function of completely removing the carrier from the test patches formed on the photosensitive belt 14. A heating roller 24 supports the function of the cleaning roller 23 by evaporating the carrier. A toner image formed on the photosensitive belt 14 is transferred to the transferring roller 25. The toner image can be transferred from the transferring roller 25 to the printing paper 27 due to a difference in surface energy. The printing paper 27 passes between the transferring roller 25 and a settled roller 26. An eraser 15 allows a new electrostatic latent image to be formed by performing a function of removing the remaining charge from the photosensitive belt 14.
The developing solution in the developers 17a, 17b, 17c and 17d is a liquid mixture of the toner and the carrier. The toner is stored in a plurality of toner containers 19a, 19b, 19c and 19d, and is supplied to a plurality of mixing containers 21a, 21b, 21c and 21d, by a plurality of pumps 20a, 20b, 20c and 20d. The carrier is stored in a carrier container 28 and is supplied to each of the mixing containers 21a, 21b, 21c and 21d. The toner and the carrier are mixed in the mixing containers 21a, 21b, 21c and 21d to an appropriate density. The toner/carrier mixtures are supplied to the developers 17a, 17b, 17c and 17d by the pumps 22a, 22b, 22c and 22d. The carrier container 28 also stores the carrier returned from a cleaning device having the cleaning roller 23 and the heating roller 24.
In the above image forming apparatus, the density of the developing solution supplied by each of the developers 17a, 17b, 17c, 17d should be maintained constant to obtain an image of an appropriate density. In other words, the mixing ratio of the toner and the carrier should be maintained at an appropriate level. The appropriate level can be realized by adjusting the amount of toner and carrier supplied respectively from the toner containers 19a, 19b, 19c and 19d, and the carrier container 28. In addition, to adjust the mixing ratio of the toner and the carrier, there should be a device for detecting the density of the image.
FIG. 2 is a view schematically showing the image forming apparatus having a conventional image density measurement device. Here, the same reference numerals are used to designate the same elements of FIG. 1.
Referring to FIG. 2, the image forming apparatus has a plurality of developing solution density adjustment devices 30a, 30b, 30c and 30d and an image density measurement device 41. The image density measurement device 41 is realized by a CTD (Color Toner Density) sensor.
The development density adjustment devices 30a, 30b, 30c and 30d maintain the density of the developing solution supplied to each of the developers 17a, 17b, 17c and 17d regularly. The density of the developing solution can be kept constant by adjusting the amount of toner and carrier respectively supplied from the toner containers 19a, 19b, 19c and 19d and the carrier container 28 to each of the mixing containers 21a, 21b, 21c and 21d. 
The CTD sensor 41 projects an infrared ray to the test patches, having tone areas different from each other and from the half tone, created on the printing paper 27. Although not shown, the CTD sensor 41 may project an infrared ray to test patches on the photosensitive medium or on a photosensitive drum. After that, the CTD sensor 41 measures the density of the image of the test patches by converting the light (a regular reflection light or a scattered reflection right) reflected from the test patches to an electrical signal by using a light-receiving element such as a photodiode. In other words, the CTD sensor 41 transmits the converted electrical signal to a controller (not shown). When the controller receives the electrical signal from the CTD sensor 41, the controller judges the density of the image corresponding to the transmitted electric signal by referring to a look-up table stored in a memory (not shown). By doing so, the density of the image is measured, and the controller can control the mixing ratio of the toner and the carrier in accordance with the measured density of the image.
However, in the test patches of the wet-type image forming apparatus using liquid toner, there is liquid carrier mixed with the toner (generally, the liquid used as the carrier is normal paraffin, but silicon oil and vegetable oil are being studied for use as a liquid carrier). Accordingly, the light projected to the test patches from the CTD sensor 41 is refracted and reflected at the surface of the test patches. The characteristic of the light which is refracted and reflected is changed as the carrier dries. In other words, the characteristic of the light is changed as a medium is added to the path of the light refracted or reflected from the test patches. In addition, since the amount of the reflected light measured in accordance with the range of the test patches is neither linear nor constant, the measured value differs from measurement to measurement. Therefore, the density of the image cannot be measured accurately.